What is a dirty joke?

A fruit seller was lying in the forest enjoying nature, and a sister went to pick mushrooms, counting "one, two, three, four, five, five, five, five, five" while picking them, and then skipped back. Guo Nan feels very comfortable today and plans to come back tomorrow.

The next day, Fruit Man continued. A bear went to the forest to pick mushrooms. While picking, count "one, two, three, four, five, five, five, six". The fruit seller ran away screaming.